Updated at 3:01 p.m. ET

Apparent Russian agents began reaching out to Donald Trump's presidential campaign as early as March of 2016, the Justice Department established in documents released Monday, with appeals for partnership and offers of help including "dirt" on Trump's opponent, Hillary Clinton.

That case is made in charging documents in the case of then-Trump foreign policy adviser George Papadopoulos, who pleaded guilty to lying to the FBI.

The court documents got a little less focus early in the day than other indictments involving Trump's one-time campaign chairman, Paul Manafort, and a top aide, Rick Gates, who was Manafort's deputy and a business partner. Manafort and Gates pleaded not guilty in federal court Monday to all the charges announced earlier Monday, NPR's Barbara Sprunt reports. The two men were deemed a flight risk, and both told the court they have a difficult time "assessing their assets at this point."

But the Papadopoulos matter speaks to the heart of the mandate for Justice Department special counsel Robert Mueller — to examine potential connections between people in the Trump campaign and Russian influence-mongers.

What is now known is that Russia tried to infiltrate the Trump campaign — and did so successfully, at least at some level. Put in context of other reporting around the Russia story, it is a remarkable establishment.

The court documents also establish that Russia promised "thousands of emails" that would have "dirt" on Hillary Clinton to Papadopoulos in April 2016. A trove of hacked Democratic emails was released by WikiLeaks three months later — in the midst of the Democratic National Convention.

"I will tell you this, Russia: If you're listening, I hope you're able to find the 30,000 emails that are missing," Trump said, appearing to encourage Russia to continue digging. His campaign denied he was doing that. "I think you will probably be rewarded mightily by our press."

Papadopoulos tried to set up additional meetings or contacts between people in the Trump campaign and Russians. There were many contacts, according to the court documents, which describe meetings or messages between Papadopoulos and at least two Russians, a "professor" in London and a woman.

But what remains unknown is who are the high-ranking campaign officials Papadopoulos contacted and what did those other officials do, if anything, with information Papadopoulos shared?

It is known, however, that Donald Trump Jr., the son of the billionaire, organized and took a meeting with Russian nationals, who also promised dirt on Clinton.

"If it's what you say," Trump Jr. wrote to the associate who set up the meeting, "I love it."

Manafort and Jared Kushner, Trump's son-in-law and a key adviser, also attended the meeting at Trump Tower in June 2016.

This is the first time a connection between the Trump campaign and Russia's attempt to interfere in the 2016 election has been established by an ironclad official government source. Reporting that has linked the Trump campaign and Russia's effort has been dismissed by the White House and the president as "fake news."

Shortly after the charges against Manafort and Gates were announced, President Trump dismissed their significance.

Papadopoulos' guilty plea is going to be more difficult for him to downplay, although the White House tried to do so on Monday.

"Today's announcement has nothing to do with the president or the campaign or campaign activity," White House press secretary Sarah Huckabee Sanders said at a White House press briefing.

Of course, that is not true. Trump named Papadopoulos as a foreign-policy adviser to the campaign. Sanders, however, dismissed him merely a "volunteer member of an advisory council that met one time."

Sanders tried to pivot to the Clinton campaign instead, alleging it "colluded" with Russian intelligence to craft the so-called "Steele Dossier." The dossier, which collected opposition research on Trump that eventually focused on his ties to Russia, was initially paid for by the conservative Washington Free Beacon website. The site is funded in large part by conservative donor Paul Singer, who was a Trump skeptic and backer of Florida Republican Sen. Marco Rubio for president.

Research for the dossier was conducted by a firm called Fusion GPS, run by two former investigative reporters. They hired Chris Steele, a former British spy turned private investigator. He has ties to U.S. intelligence and is known as a Russia expert. As a specialist, they hired him to find more information about Trump's ties in Russia that they couldn't get.

When the primaries were over, the Clinton campaign and the Democratic National Committee picked up the tab to keep the research going.

Sanders called the dossier as "false information." Some of it has been confirmed by some news outlets. Some of it is incendiary and has not been independently confirmed. Sanders dismissed the meeting at Trump Tower between Trump Jr. and Russian nationals as simply "a meeting that took place" and "routine."

It was not routine. Seeking information you are is coming from a foreign government in an effort to help your campaign is unlike little else known publicly about how campaigns are conducted in the United States.

Sanders, though, contended the Clinton campaign's and DNC's "millions of dollars" paid for the oppo-research file (that it never used) was far worse that Trump Jr.'s meeting — or anything coming out of Monday's court documents.

"They took one meeting, and nothing came of it," she said.

Sanders also said the president had "no reaction," because "it doesn't have anything to do with us."

Not the end of the Mueller investigation

There could be more details to come either from Mueller or the congressional committees that are investigating the Russia imbroglio. Court documents indicate that Papadopoulos has met with the government "on numerous occasions to provide information and answer questions."

Sen. Mark Warner of Virginia, the top Democrat on the Senate Intelligence Committee, which is also investigating Russia-Trump ties, called Monday's developments in a statement "a significant and sobering step in what will be a complex and likely lengthy investigation by the Special Counsel. That is why it is imperative that Congress take action now to protect the independence of the Special Counsel, wherever or however high his investigation may lead."

His office also said he wants bipartisan members of Congress to "make clear" to President Trump "that issuing pardons to any of his associates or to himself would be unacceptable, and result in immediate, bipartisan action by Congress."

The president has the unfettered ability to pardon anyone from federal crimes. But that only applies to federal crimes. Some officials, including Manafort, could also face charges at the state level — in New York, for example.

Warner also put the guilty plea and indictments in the context of broader developments on the Trump-Russia investigation over the last several months.

"This is just the latest in a series of undisclosed contacts, misleading public statements, potentially compromising information, and highly questionable actions from the time of the Trump campaign that together, remain a cause for deep concern and continued investigation," he said.

And there are indications that Papadopoulos is cooperating with federal agents.

"It is in the best interest of our client, George Papadopoulos, that we refrain from commenting on George's case," his lawyers, Thomas M. Breen and Robert W. Stanley, said in a short statement obtained by NPR's Carrie Johnson.

They then added: "We will have the opportunity to comment on George's involvement when called upon by the Court at a later date. We look forward to telling all of the details of George's story at that time."

The charges

Manafort and Gates, were charged with "conspiracy against the United States," "conspiracy to launder money" and other offenses. The two were expected in court in Washington by the afternoon.

The Justice Department indictment on Manafort and Gates contains 12 counts: "conspiracy against the United States, conspiracy to launder money, unregistered agent of a foreign principal, false and misleading FARA statements, false statements, and seven counts of failure to file reports of foreign bank and financial accounts."

Papadopoulos pleaded guilty to making false statements to the FBI.

The Manafort and Gates indictment unsealed on Monday morning does not make any reference to Russia's influence campaign against the presidential election, but it does allege extensive financial ties between Manafort and Gates and powerful Ukrainians.

The Papadopoulos materials, on the other hand, detail the many contacts investigators say he had with Russian-linked operatives. He met at least two people, a man and a woman, who the FBI says were working for the Russian government and had boasted to him about the help it could offer the Trump campaign against Clinton.

Manafort and Gates turned themselves in to the FBI on Monday morning. Papadopoulos' status was unclear.

Manafort appeared at the FBI's Washington field office just after 8 a.m. with his lawyer. He was escorted into the building by an FBI agent.

A spokesman for the special counsel declined to comment to NPR.

The materials are online here.

The Papadopoulos court documents are available here.

The Ukraine connection

Manafort headed Trump's campaign from June to August of 2016. He stepped down after reports emerged about his business relationship with pro-Russian leaders in Ukraine, allegedly replete with millions of dollars in cash payments and undisclosed lobbying efforts.

The indictment released on Monday charges Manafort and Gates with the extensive use of offshore bank accounts, through which flowed more than $75 million. The document alleges that Manafort laundered more than $18 million of that to conceal it from U.S. authorities and Gates transferred more than $3 million.

Manafort grew wealthy over a colorful career in his four decades as a Republican operative and lobbyist.

He worked for Republican campaigns in the late 1970s and early 1980s before moving into the lucrative world of international lobbying, where he teamed up with Roger Stone to found the firm Black, Manafort, Stone & Kelly.

Manafort represented a host of unsavory characters over the years, including Angolan rebel leader Jonas Savimbi, Philippines President Ferdinand Marcos and Zaire's leader Mobutu Sese Seko.

In the early 2000s, Manafort headed to Ukraine to work as an adviser to the pro-Russian politician Viktor Yanukovych, helping him win the presidency in 2010. Yanukovych was overthrown by a wave of popular protests four years later. He fled to Russia.

Manafort later became an early backer of Trump, eventually assuming the role of chairman of his insurgent presidential campaign. Trump and the White House have lately downplayed the role that Manafort played in the 2016 operation. But at the time, their relationship was said to be close.

Manafort is "the one person in the room that calls him 'Donald,' " as Republican operative Scott Reed told NPR last year. "It's not 'Mr. Trump.' It's 'Donald.' 'Come on, Donald; we've got to do the right thing here."

A spokesman for Manafort did not respond to a request for comment.

Read the full Manafort indictment here.

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